1. Field of the Invention
This invention relates to a disk system in which a disk used as a recording medium can be replaced and to a recording medium and a disk drive for recording an audio signal in the form of a digital signal.
2. Description of the Related Art
With the formation of a computer system of even greater miniaturization and higher performance, requirements for the large capacity, high speed, miniaturization, light-weight and low cost of the removable magnetic disk system have become further stronger. In general, the removable magnetic disk system may be roughly divided into three types, a floppy disk drive (FDD), a removable hard disk drive (HDD), and a Bernoulli flexible disk drive. Among them, the FDD is widely used because of its low cost and easy handling.
However, since the FDD has a small capacity and low rotation speed, the data transfer speed and access speed will become low. One of the main reasons why the capacity is small is that the conventional FDD is driven according to the non-tracking servo system and the operation thereof cannot follow the expansion and contraction of a flexible medium due to the temperature and moisture so that the track density cannot be increased. However, recently, a high track density recording technique has been developed by use of a tracking servo system. Further, the above problem is caused by a difference between the magnetic head systems. The rotation speed of the FDD is normally 300 to 360 rpm and is lower than that of the HDD by one digit. This is because the head is used in contact with the medium and it is impossible to attain a long service life of the medium if the rotation speed of the FDD is set as high as that of the HDD in which the head is set apart or floated from the medium.
Initially, there are two types of double-sided head systems of the FDD. One type of the head system is a head suspension system of the HDD and can be obtained by modifying IBM3330 for the FDD service and it is disclosed in U.S. Pat. No. 4,263,630.
The other type of the head system is proposed by Tandon Co. and one of the double-sided heads is fixed and the other head is formed with a gimbal structure. This type of head system is disclosed in U.S. Pat. No. 4,151,573.
Double-sided head systems of the FDD which are now widely used are constituted to attain the "head touch" on both sides of the medium by setting the medium in contact with sliders provided on both sides thereof. Therefore, large friction occurs between the head and the medium and a spindle motor for rotating the disk is required to have large torque. Thus, it is understood that the above head system is not preferable from the viewpoint of the durability of the medium.
The mechanical size and mass of the head system of the FDD are larger than those of the head system of the HDD. Therefore, the movable portion of the head positioner becomes large and the mass thereof becomes large so that it will be difficult to attain a high seek speed.
The double-sided magnetic head system of the FDD maintains the head touch by holding the medium by use of the head sliders. The head gap lies at substantially the central portion of the slider. With the above bulk type magnetic head, it is difficult to precisely control the track width and gap length in the manufacturing process. For this reason, even if the positioning error can be reduced by use of the tracking servo system, it is difficult to enhance the track density because the track widths of the devices are different. Further, there occurs a problem that large OWM (Over Write modulation) noises due to the difference in the gap length are generated.
The removable HDD is advantageous over the FDD in that a large capacity and high-speed access can be attained, but at the same time it has disadvantages that the disk cartridge is expensive and there is a possibility that head crash may be caused by dust, adhesion, mechanical shock and the like. Further, since the head is set apart or floated from the disk when it is in use, spacing loss occurs, thereby making it impossible to enhance the linear recording density. Generally, the most serious problem of not only the removable HDD but also the other HDD is the head crash and the problem occurs as an essential defect of the HDD caused by the rigidity of the disk itself. Particularly, in the removable HDD, a larger amount of dusts are introduced into the HDA (head disk assembly) in comparison with a case of the fixed type HDD and therefore the possibility of head crash becomes higher accordingly.
The head assembly of IBM3370 type is simple in construction and the rigidity of the suspension is high in comparison with a head assembly shown in FIG. 20.
Generally, it is preferable to set the floating distance as small as possible from the viewpoint of high-density recording, but if the floating distance is set excessively small, the head comes into contact with the disk surface and the durability of the medium cannot be kept high. Further, adhesion occurs in CSS (Contact Start Stop) when the profile irregularity is improved.
As described above, since the head of the hard disk is floated with an extremely small floating distance, head crash may be caused when dust is introduced between the disk and ABS (Air Bearing Slider) even if the size of the dust is of the order of submicrons.
The IBM3370 type head assembly sufficiently follows the surface deflection of the hard disk and still exhibits high rigidity. Therefore, high track density can be easily attained by the tracking servo in the HDD. However, the head crash due to dust develops into a further serious problem in the removable HDD in comparison with the normal fixed type HDD. In the case of the removable HDD, a hermetically sealed structure for preventing dust from entering the cartridge is used, but since the shutter is opened or closed for head loading, it is difficult to prevent entrance of dust of the order of submicrons. Further, since a cartridge having dust adhered thereon is loaded into the drive, it is impossible to clean the internal portion of the HDA (head disk assembly) unlike the fixed type HDD. Therefore, the removable HDD is less reliable than the fixed type HDD, and if the problem associated with dust cannot be solved, the floating distance cannot be reduced, thereby making it impossible to enhance the recording density.
Then, a Bernoulli flexible disk drive has been proposed as a device in which the disk is rotated at a high speed and can be replaced. The basic construction thereof is described in U.S. Pat. No. 4,419,704. Since the head is kept apart from the medium in the Bernoulli type disk drive, the rotation speed can be easily enhanced. However, since a floating gap is made between the head and the medium, a problem that the recording density cannot be enhanced like the normal HDD occurs. Since a flexible disk is used, it is highly resistant to head crash which will occur in the HDD, but the medium will be easily damaged like the HDD when dust is introduced. For this reason, the same filtration as in the removable HDD becomes necessary and the cost thereof becomes high. Further, the head is complicated in construction in comparison with the heads of the conventional FDD and HDD and the cost of the device becomes high. Further, since an R/W gap lies in the central portion of the spherical slider, a thin film head such as the head of the HDD cannot be used. Since the Bernoulli type device is basically a single-sided head type, two disks must be inserted into a single cartridge when a double-sided head is simulatively formed and the disk cartridge becomes expensive.
A digital audio medium may be mainly divided into two types based on the length of time of performance, one type being a compact disk (CD) exclusively used for reproduction and the other type being a digital audio tape (DAT) used for recording and reproduction. In addition to the above two types of media, there is provided a 3.5" floppy disk (3.5" FD) whose recording capacity is small but which is used in personal computers, word processors or musical instruments. Widely used 3.5" FDs are of 2DD and 2HD which use a Co-.gamma. medium and whose unformatted capacities (e.g. UF capacities) are respectively 1 MB and 2 MB. Recently, floppy disks of 2ED (i.e. UF capacity is 4 MB) using a Ba-ferrite vertical recording medium are commercially available and the capacity of the 3.5" FD is rapidly increased. The above driving devices are compatible with lower FDs for read/write.
Further, recently, an FD of large capacity having a formatted capacity (e.g. FT capacity) of 10 MB or more obtained by using tracking servo (which is sector servo in most cases) in the FD so as to enhance the track density to four times that of the conventional case has been experimentally manufactured and developed.
As described above, with popularization of the CD, it becomes natural to convert an audio signal into a digital form. Naturally, the requirement of the users that the audio signal can be easily recorded in a digital form becomes stronger, but the CD, DAT or 3.5" FD cannot fully meet the requirement of the users at present. That is, the medium available at present does not have such a performance and function as to satisfy the users in terms of the size, the easy handling, the degree of protection from dusts, damages and rough handling, the easy transportation, the recording function, the random access function, the repetitive durability, the recording time, the price and the like.
In general, the CD is excellent as a source of digital sound of high quality but has an essential defect that no recording function is provided. Further, it has defects associated with the size, the handling, the transportation, the price and the like. Particularly, the difficulty in handling the same caused by the fact that it is not set inside a cartridge, the difficulty of removing the same from the case, the degree of protection from dust, damage and rough handling are serious problems.
The DAT somewhat compensates for the defects of the CD that no recording function is provided and the handling is difficult, but problems associated with the infringement of copyright of a digital copy and the high price thereof are provided and there is no tendency that it is widely used. The other problems inherent to the DAT are that the "repetitive durability" and "random accessibility" are low and the degree of protection from the dust, damage and rough handling is extremely low because of an incomplete cartridge structure.
Widely used audio media (i.e. audio tapes or the like) have various problems. Since a 3.5" FD is set inside a proper cartridge, it is easily handled and is excellent in the repetitive durability and random accessibility and the price thereof is low. However, the capacity thereof is not large enough to record a sufficiently large amount of audio signals in the digital form. The other problem of the 3.5" FD is that (1) it is impossible to completely prevent dust entrance because there is a gap near the chucking hub, (2) there is a high possibility that dust attached to the shutter are dropped off onto the disk by the shuttering operation, and (3) the medium is easily exposed and may be easily damaged since a lock mechanism of the shutter is not provided.
As described above, it is understood that the removable disk drives have various types of problems. That is, the problems are as follows:
For example, (1) in the case of a floppy disk, problems that the capacity thereof is small and the rotation speed thereof is low are provided.
(2) In the case of a removable HDD, problems associated with an expensive disk cartridge, head crash by the presence of dust, impossibility of enhancing the recording density are provided.
(3) In a Bernoulli type device, in addition to the problems similar to those of the HDD except the problem associated with head crash, there are further provided problems that an expensive head which is complicated in construction is required and two disks are necessary when a double-sided head is simulatively formed.
Further, the above-described 3.5" FDs have various problems. In particular, when a digital audio signal is recorded on the FD, problems that (4) the recording operation can be effected only for approx. 70 seconds and that (5) the recording operation cannot be effected while permitting the user to freely control (i.e. select) the quality of the recording signal even if the recording operation can be effected for several tens of minutes the serious problems.